A known electronic engine control system comprises a processor-based engine controller that processes data from various sources to develop control data for controlling certain functions of the engine, including fueling of the engine by unit fuel injectors that inject fuel directly into engine cylinders. One type of unit fuel injector is commonly known as a HEUI injector, the four-letter acronym standing for hydraulically-actuated, electrically-controlled unit injector.
A HEUI injector has a fuel inlet port communicated to a source of fuel under pressure, such as pressurized fuel in a fuel rail. It also has an oil inlet port communicated to a source of hydraulic fluid under pressure, such as pressurized oil in an oil rail. Fuel is injected out of the injector and into an engine cylinder through orifices in the tip end of a nozzle disposed within the head end of the engine cylinder.
Injection of fuel is controlled by an electric-actuated control valve that when actuated open allows oil from the oil rail to pass through the oil inlet port to an oil outlet port for applying hydraulic force to a piston that is disposed at one end of a plunger. The piston transmits the hydraulic force to the plunger, and because the plunger has a smaller diameter than the piston, the pressure that the plunger applies to fuel that has entered from the fuel rail is intensified. The intensified pressure acts on certain movable elements within the fuel injector.
One such movable element is a fuel inlet check; another is a reverse flow check.
When the control valve is actuated open, the intensified pressure applied by the plunger to fuel that has entered by passing through the fuel inlet port and into a bore within which the plunger is guided forces the fuel inlet check to close the fuel inlet port so that the fuel cannot backflow out through the fuel inlet port. The intensified pressure also forces open the reverse flow check to enable the intensified pressure to unseat a spring-biased needle from an internal seat in the nozzle. The unseating of the needle against an opposing force of a needle bias spring opens a high-pressure injection path from the plunger to the nozzle orifices to force fuel through that path and be injected out of the orifices and into an engine cylinder as the plunger is forced to extend through the bore by the that oil passes through the control valve.
When the control valve is actuated closed, the flow of oil stops, greatly attenuating the pressure that the plunger was applying to the fuel. A return spring that was being compressed as the piston and plunger were extending to force the fuel injection now forces the piston and plunger to retract. This causes the needle bias spring to re-seat the needle and thereby terminate the fuel injection. The reverse flow check closes to keep some pressure in fuel between it and the nozzle, and the fuel inlet check opens to allow fuel to enter and refill the fuel injector as the plunger retracts.
Performance of the fuel injector is dependent upon various factors, one of which is the speed at which the actuator can open and close the oil inlet port.